The present invention generally relates to High Intensity Discharge (HID) lamps and more particularly to a system and method for preventing acoustic arc resonance in HID lamps by utilizing a pulse phase modulation (PPM) switching power source.
HID lamps are widely used for outdoor lighting because of their high efficiency and long life. However, several HID lamp characteristics affect conventional HID ballast costs and reliability. Conventional HID lamps operate at low working voltages from 50V to 100V and high ignition voltages greater than 1500V. HID lamps operate at very low voltages of less than one third of the normal operating voltage after ignition until the HID lamp reaches its full power. Normally this period lasts several minutes. This characteristic requires that HID ballasts be able to withstand over load.
HID lamps further are characterized by highly dynamic characteristic loads which change instantly from low impedance to open circuit or from open circuit to low impedance. Therefore HID ballasts must have a constant power output with robust open load and over load protection.
Acoustic arc resonance (AAR) can affect HID lamp stability and damage the HID lamp. AAR can happen from several hundred Hz to several hundred KHz. The AAR may vary from HID lamp to HID lamp and may change as the HID lamp ages. AAR is not a problem for magnetic HID ballasts that work at 50 to 60 Hz power line frequency. However electronic ballasts that operate within this frequency range must deal with AAR.
Magnetic ballasts use large power transformers to handle overload during the HID lamp ignition with the attendant costs of low power factor, heavy weight, and low efficiency. Normally an expensive power capacitor is used in magnetic HID ballasts to compensate for the low power factor. High efficiency electronic HID ballasts are more efficient than magnetic ballasts as they generally are smaller, weigh less, are more efficient, and have a higher power factor. In order to replace magnetic ballasts, high efficiency electronic HID ballasts must effectively control AAR and have robust power load and over load protection achieved at a reasonable cost.
Conventional high frequency electronic HID ballasts use a pulse frequency modulation (PFM) method to prevent AAR. The major problem with PFM is acoustic noise and electromagnetic noise. The rapid frequency jumps in PFM can effectively prevent AAR but also increase the noise level. Slower frequency jumps can reduce the noise level but may not completely prevent MR. Some high frequency electronic HID ballast designs use a fast AR feedback circuit to trigger the frequency jump whenever AAR is detected. This approach can minimize acoustic noise, but introduces additional costs. The PFM method also increases ballast design difficulty because the ballast circuit needs to be optimized for multiple working frequencies.
As can be seen, there is a need in the art for a high frequency electronic HID ballast having open load and over load protection that is reliable and low cost. Preferably the high frequency electronic HID ballast operates at a low ignition frequency to extend HID lamp and starter life and to offer open load protection to the starter. Further, the high frequency electronic HID ballasts preferably operates at fixed frequency with PPM to prevent AAR.